A non-contact communication system consists of an interrogator installed in a fixed position and a responder formed in the shape of a card so as to be portable. The supply of electric power from the interrogator to the responder and the communication between the interrogator and the responder are achieved by electromagnetic induction on a non-contact basis. Thus, non-contact communication systems, having the advantages that the responder does not need to incorporate a battery and that the interrogator and the responder communicate with each other on a non-contact basis, are adopted in a variety of fields, for example in lift facilities in skiing grounds, for ticket examination in railway systems, and for baggage sorting.
Now, an interrogator 1 and a responder 2 constituting a non-contact communication system will be described with reference to FIG. 7. The interrogator 1 operates as follows. A modulator circuit 12 modulates a carrier, which is a radio-frequency signal output from a carrier supply circuit 11, with an interrogation signal Q, which is fed from a controller 16, and outputs a modulated signal. The signal output from the modulator circuit 12 is subjected to power amplification by a power amplifier circuit 13, and is then transmitted from an antenna 14 composed of a coil 141 and a capacitor 142.
The controller 16 is so configured as to keep feeding the interrogation signal Q to the modulator circuit 12 for a predetermined period, then cease feeding it thereto for a predetermined period, and then repeat these two modes of operation alternately. Thus, as shown in FIG. 8(a), the signal transmitted from the interrogator 1 is a radio-frequency signal that is modulated with the interrogation signal Q in periods T1 and that is unmodulated in periods T2, with the periods T1 and T2 repeating alternately.
The responder 2 operates as follows. Through electromagnetic induction, the signal transmitted from the interrogator 1 is received by an antenna 21 composed of a coil 211 and a capacitor 212. The signal received by the antenna 21 is rectified by a rectifier circuit 22, and a regulator 25 produces, from the resulting direct-current electric power, stabilized direct-current electric power. A demodulator circuit 26 and a signal processor circuit 27 operate from the electric power produced by the regulator 25.
The demodulator circuit 26 demodulates the interrogation signal Q from the signal obtained from the rectifier circuit 22 in the periods T1. The signal processor circuit 27 outputs, in the periods T2, a response signal A corresponding to the interrogation signal Q demodulated by the demodulator circuit 26. This response signal A consists of a train of pulses, and controls the on/off states of a switch circuit 24. The switch circuit 24 is connected in series with a resistor 23, and these are connected across the output of the rectifier circuit 22. Thus, as the switch circuit 24 is turned on and off, the impedance of the load circuit that is connected to the output of the power amplifier circuit 13 of the interrogator 1 and that thus includes the responder 2 varies. The resistor 23 and the switch circuit 24 may be connected across the input of the rectifier circuit 22.
Accordingly, in the periods T2, the signal output from the power amplifier circuit 13, which is originally an unmodulated carrier, is amplitude-modulated by the response signal A as shown in FIG. 8(b) as a result of the variation in the impedance of the load circuit of the power amplifier circuit 13. This modulated signal is demodulated by a demodulator circuit 15 of the interrogator 1, and thus the response signal A is obtained. According to the response signal A obtained from the demodulator circuit 15, the controller 16 operates in a predetermined manner.
In general, unnecessary electromagnetic emission from electric equipment is alleviated by matching circuits, or by providing a shield made of metal or the like. However, in the non-contact communication system described above, the interrogator and the responder form a complete circuit with space interposed in between, and therefore it is difficult to achieve circuit matching. On the other hand, by shielding the whole interrogator, as by forming the housing thereof out of metal or the like, it is possible to alleviate unnecessary electromagnetic emission, but this makes communication impossible. For these reasons, it is customary, as a measure to alleviate unnecessary electromagnetic emission from the interrogator 1, to enclose only the internal circuits 10 thereof, i.e. the electric circuits other than the antenna 14, in metal or the like.
However, in the conventional interrogator, almost no measures are taken against unnecessary electromagnetic emission from its antenna itself, and thus unnecessary electromagnetic emission from the interrogator as a whole is not satisfactorily alleviated.